In recent years, by the development of remote sensing technologies, observational instruments mounted in aircraft and artificial satellites have grown in performance, and the amount of information transmitted from the air to the ground is increasing. In order to cope with further performance improvement of the observational instruments in the future, data communication technologies that use free space optics (FSO) have been aggressively studied that uses optical frequency bands unconstrained by radio frequency bands. The achievement of the high-capacity free space optics (FSO) requires a high-speed technology for a transmission rate and a wavelength multiplexing technology. In this case, it is efficient to use a common technology with an optical fiber communication technology, that is, to apply an optical transmitting and receiving technology using a single mode fiber (SMF).
Examples of a free space optical communication device using the optical fiber communication technology are described in Patent Literature 1 and Patent Literature 2.
In the free space optics (FSO) technology, it is general to make a modulated laser beam with a narrow beam to propagate through the air. On the receiving side, light beams are collected by an optical antenna and propagate through a short-range fiber, and then signal reception is performed.
In a free space optics (FSO) receiver, a wave-front distortion of laser light due to atmospheric propagation becomes a problem as described below. A beam spot is formed on a focal plane in a collecting unit of the free space optics (FSO) receiver, and a speckle pattern arises on the beam spot due to an atmospheric disturbance. By the occurrence of the speckle pattern, the beam spot diffuses or moves (scintillation) against an ideal focal plane.
In the free space optics (FSO) receiver, optical coupling with a single mode fiber (SMF) is required as a bit rate of a signal increases; however, the above-described phenomenon of beam spot variation becomes a serious problem because it brings deterioration of the coupling efficiency. The reason is that the speckle pattern has a relatively significant impact on the single mode fiber (SMF) having a small core diameter; accordingly, the loss of received data occurs due to a slight scintillation, and an effective communication rate decreases.
In order to prevent the deterioration of the coupling efficiency with the single mode fiber (SMF) described above, the free space optical communication device described in Patent Literature 1 is configured to use a fiber cable bundling a plurality of optical fibers tightly, for example, an optical fiber bundle. Specifically, the free space optical communication device described in Patent Literature 1 includes a precise acquisition and tracking function section having a convex lens as a light collection optical system into which the light beam from a communication opposite station received by a transmitting and receiving telescope is introduced, and a fiber cable bundling a plurality of optical fibers tightly. The precise acquisition and tracking function section forms a light focus of the light beam having been transmitted through the convex lens on a light transmission/reception surface of one end of the fiber cable where ends of first to n-th optical fibers are exposed, and couples the incident light with at least one of the optical fibers. This makes a configuration in which the received light is guided through any one of the first to n-th optical fibers.
Patent Literature 2 discloses a FSO receiver that uses a single fiber tapered from a large core to a small core instead of a fiber bundle. Specifically, the FSO receiver described in Patent Literature 2 includes a telescopic collection system, a wavelength demultiplexer, photodetectors, analog-to-digital converters, and a digital signal processor. The FSO receiver has a configuration in which a tapered fiber bundle or a tapered single fiber collects light from the demultiplexer into a plurality of individual fiber endfaces and concentrates it into a single output fiber for input to the photodetector.
By this means, a relatively large optical aperture is provided for collecting the optical signal. Thus, there are known tapered fiber bundles that employ an adiabatic taper to efficiently couple the collected light into a single-mode output fiber for efficient detection. It is said that one benefit of the large aperture that is afforded by the above-described technique is greater tolerance to beam wander which tends to degrade the performance of the communication system.